Every time you turn around, a presidential candidate whips out his Bible—or a position paper—to let us know how faithful he or she is. Senator John Edwards (D-N.C.) says God "would be happy with the fact that" he's focused on people without health care. Senator Barack Obama (D-Ill.) says we should "discuss religion . . . in the positive sense of what it tells us about our obligations towards one another." Republicans, also, are quick to point out how faith informs their policies.

Clearly, the candidates are appealing to America's religious voters—and they are smart to do so. As one social scientist recently noted, they are going to need religious voters for the long term—because Christians are having far more children than their secular neighbors.

Writing in the Wall Street Journal, social scientist Arthur Brooks notes that if you pick 100 adults out of the population who attend their houses of worship nearly every week, they would have 223 children among them. But among 100 people who attend religious services less than once a year—or never—you would find 158 kids. That's a 41 percent fertility gap between religious and secular people.

Even worse—if you are a secularist—religious people who identify themselves as politically "conservative" or "very conservative" are having, on average, an astonishing 78 percent more kids than secular liberals, Brooks writes.

This is significant, because kids tend to grow up to worship the way their parents do. In a generation or two, we are going to have a bumper crop of conservative citizens. Candidates who appeal to Christians will win more elections simply because of demographics.

This is not the first time in history we have seen the demographic power of the Church. Take, for example, ancient Rome.

In his book The Rise of Christianity, Rodney Stark describes ways in which religious belief affected population growth and decline. Pagans believed in abortion; Christians forbade it. Christianity also prohibited infanticide. But as for pagans, Stark told Touchstone magazine: "We've unearthed sewers clogged with the bones of newborn girls." So, "Christians didn't have the enormous shortage of women that plagued the rest of the empire."

And pagan husbands engaged in adultery, polygamy, and divorce, but Christianity forbade these things. So, Christian marriages tended to be more loving, more faithful, and more open to children.

We see parallels today between the modern West and ancient Rome. The low pagan fertility rate meant that Rome needed to import workers—and soldiers—from the farthest reaches of the empire and beyond. Rome lost its social cohesion. So, Rome fell. The Christian Church survived because believers had been multiplying. And what is going to cause Christianity to recover in the modern West may be precisely the same phenomenon.

For the last half century, Western industrialized nations, fearing overpopulation and despoiling the planet, have made slowing population growth one of their top priorities. So now we are in the middle of what one observer calls a "global baby bust"—except, that is, among devout Christian families, those who take seriously the biblical mandate to "be fruitful and multiply." It shows that when Christians live out the biblical worldview, we not only survive, we thrive.